Colon cancer is the third most common and second deadliest cancer in the US, with 135,000 new cases diagnosed and 56,000 deaths each year. Most colorectal cancers begin as benign polyps that produce few symptoms but can be found in screening examinations. Since most polyps grow very slowly, deaths from colon cancer could be markedly reduced with effective screening of the population over 50 years old. Common screening options include barium enema, colonoscopy, and, increasingly, CT colonography. Colonoscopy is considered the gold standard, and involves passing an endoscope through the entire colon. It has high sensitivity (˜95%), and has the advantage that polyps, if found, can be removed during the examination. However, it requires sedation, and there is a finite chance of bowel perforation (2 in 1000) or even death (1 in 19000). It also requires complete bowel preparation. Typically, this involves up to two days of fasting drinking only clear liquids, and ingestion of large amounts of laxative in the 24 hours prior to the exam to cleanse the colon as completely as possible. The inconvenience and discomfort associated with this preparation is an important obstacle to compliance with currently recommended colorectal screening guidelines. It is estimated that only 40% of the population above 50 years old undergo any kind of colon screening, and only 25% undergo a full colorectal structural exam. By contrast, 85% of women in the appropriate age group undergo mammography. Patient surveys show that the necessity for bowel preparation is cited as the main reason for avoiding colon examination.
CT colonography (CTC) is being extensively studied as an alternative for colon examinations, since it offers many advantages such as lower risk and less patient discomfort. By way of example, these technologies are disclosed in the Johnson et al. U.S. Pat. Nos. 6,928,314 and 7,035,681, the Zalis U.S. Pat. No. 6,947,784, the Vining U.S. Pat. Nos. 6,909,913 and 7,149,564 and PCT publication no. WO 2007/030132, all of which are incorporated herein by reference. Briefly, CT data covering the entire colon is acquired and examined on a computer workstation, either paging through 2D slices with orthogonal views and 3D rendering available, or using a full 3D endoscopic fly-through of the colon.
CTC as typically performed often requires complete bowel preparation. If the onerous bowel preparation could be eliminated, it can be expected that patient compliance would dramatically improve. However, in an unprepared colon residual stool and fluid can mimic soft tissue density and thus confound the identification of polyps. Various groups have proposed tagging the stool with an opacifying agent so that it is brighter than soft tissue and thus easily recognized automatically. Pixels identified as stool can then be electronically subtracted away by being reset to air values. Most commonly, this is done in conjunction with partial bowel preparation, such as a low-fiber or liquid diet for 24-48 hours before the exam.
Enhanced patient compliance may be achieved when the inconvenience and discomfort to the patient is as small as possible. Accordingly, there is ongoing research involving unprepared CT colonography, with no dietary restrictions, no laxatives, and simple ingestion of oral barium and iodine beginning 48 hours before the exam. This approach leads to colonic content that can be solid, liquid, or semi-liquid. Stool tagging is usually homogenous, especially past the ascending colon.